1. Field of the Invention
The field of the invention relates generally to Radio Frequency Identification (RFID) systems and more particularly to systems and methods for reading and writing information from multiple RFID enabled documents.
2. Background of the Invention
FIG. 1 illustrates a basic RFID system 100, A basic RFID system 100 comprises three main components: an antenna or coil 104; an interrogator 102, and a transponder, or RF tag 106 which is often electronically programmed with unique information. Antenna 104 can be configured to emit radio signals 108 to activate tag 106 and read and write data from the activated tag 106. Antenna 104 is the conduit between tag 106 and interrogator 102, which is typically configured to control data acquisition and communication. Antennas 104 are available in a variety of shapes and size. For example, in certain embodiments they can be built into a door frame to receive tag data from persons or things passing through the door. In other embodiments, antennas 104 can, for example, be mounted on an interstate toll booth to monitor traffic passing by on a freeway. Further, depending on the embodiments, the electromagnetic field, i.e., radio signal 108, produced by an antenna 104 can be constantly present when, e.g., multiple tags 106 are expected continually. If constant interrogation is not required, then radio signal 108 can, for example, be activated by a sensor device.
Often antenna 104 is packaged with interrogator 102. A conventional interrogator 102 can emit radio signals 108 in ranges of anywhere from one inch to 100 feet or more, depending upon the power output and the radio frequency used. When an RFID tag 106 passes through an electromagnetic zone associated with radio signal 108, it detects radio signal 108, which can comprise an activation signal. In some embodiments, interrogators can comprise multiple antenna, though typically only one transmits at a time.
Additionally, interrogator 102 is often coupled through network 112 to a central server 112. Central server 112 can be configured to execute a number of applications including those which incorporate data from an RFID tags. For example, in a tracking system, interrogator 102 transmits to the central server 112 the identity of tags which pass through its interrogation zone. This information can be correlated to objects associated with the tag in a database residing on the central server and hence the whereabouts of the object in question at that particular time can be logged. In the example of a toll booth, tags that pass through the specific toll both are reported to central server 112 which correlates the tag to a motorist who is then debited the cost of the toll.
RFID tags 106 come in a wide variety of shapes and sizes. Animal tracking tags, for example, inserted beneath the skin, can be as small as a pencil lead in diameter and one-half inch in length. Tags 106 can be screw-shaped to identify trees or wooden items, or credit-card shaped for use in access applications. Anti-theft hard plastic tags attached to merchandise in stores can include RFID tags. In addition, heavy-duty RFID tags can be used to track intermodal containers, heavy machinery, trucks, and/or railroad cars for maintenance and/or tracking purposes.
RFID tags 106 are categorized as either active or passive. Active RFID tags 106 are powered by an internal battery and are typically read/write, i.e., tag data can be rewritten and/or modified. An active tag's memory size varies according to application requirements. For example, some systems operate with up to 1 MB of memory. In a typical read/write RFID work-in-process system, a tag 106 might give a machine a set of instructions, and the machine would then report its performance to tag 106. This encoded data would then become part of the tagged part's history. The battery-supplied power of an active tag 106 generally gives it a longer read range. The trade off is greater size, greater cost, and a limited operational life.
Passive RFID tags 106 operate without a separate external power source and obtain operating power generated from radio signal 108. Passive tags 106 are consequently much lighter than active tags 106, less expensive, and offer a virtually unlimited operational lifetime. The trade off is that they have shorter read ranges than active tags 106 and require a higher-powered interrogator 102. Read-only tags 106 are typically passive and are programmed with a unique set of data, usually 32 to 128 bits, that cannot be modified. Read-only tags 106 often operate as a license plate into a database, in the same way as linear barcodes reference a database containing modifiable product-specific information.
RFID systems are also distinguishable by their frequency ranges. Low-frequency, e.g., 30 KHz to 500 KHz, systems 100 have short reading ranges and lower system costs. They are commonly used in security access, asset tracking, and animal identification applications. High-frequency, e.g., 850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz 100 systems offer long read ranges, e.g., greater than 90 feet, high reading speeds, and are used for such applications as railroad car tracking and automated toll collection, however, the higher performance of high-frequency RFID systems 100 incurs higher system costs.
The significant advantage of all types of RFID systems 100 is the noncontact, non-line-of-sight nature of the technology. Tags 106 can be read through a variety of substances such as snow, fog, ice, paint, crusted grime, and other visually and environmentally challenging conditions, where barcodes or other optically read technologies cannot typically be used. RFID tags 106 can also be read in challenging circumstances at high speeds, often responding in less than 100 milliseconds. RFID has become indispensable for a wide range of automated data collection and identification applications that would not be possible otherwise
In other RFID systems the tags can remain relatively stationary. For example, in a warehouse tracking application, a forklift can be equipped with an RFID interrogator, whose position and other motion information can be determined by reading RFID tags on the floor of the warehouse. In this application, the RFID tags are permanently affixed to the floor of the warehouse deployed in a known arrangement where the position of the tags are known ahead of time.
In a shipment tracking application, a handheld RFID interrogator can be passed over a package to read an RFID tag. Typically, the operator can scan a package's RFID tag 106 by passing the interrogator 102 with the antenna 108 mounted on it or just the antenna 108 near the RFID tag 106. Though the package can be mobile as well, the interrogator 102 or its antenna 108 are easily moved by the operator.
Still other RFID systems can have both stationary RFID tags 102 interrogated by stationary interrogators 102. For example, in a medical inventory system, medication in containers with RFID tags 106 affixed to them are placed inside a cabinet or drawer. When the cabinet is closed or locked, an RFID interrogator 102 takes an inventory of the contents of the cabinet. A central server 112 can compare the results of this inventory to that of the inventory prior to the opening of the door, which yields a list of medication that was either added or removed from the cabinet. Such a system can be used to insure that the proper medication for a specific patient is removed for use.
An additional challenge in RFID interrogation arises when used in an environment where both the antenna and the RFID tags are stationary. In normal transmission of electromagnetic energy, reflections from objects, RFID tags can cause destructive interference leading to regions in the electromagnetic fields with either little or no energy. In addition, in systems where multiple antenna are used either by the same interrogator or by a second interrogator. The electromagnetic fields generated by these antennae can also destructively interfere leading to regions of little or no energy. In RFID applications where the tags are passed through a field, this phenomenon is not a problem since the tags are moved through the field and only resides in one of these no energy regions for a very brief period of time. Likewise, when the interrogator or interrogator's antenna is mobile, the field is then moved about the tags so these no energy regions are moved around the tags.